T-Mobile US (TMUS) plans to turn down Iliad's $15 billion deal proposal, sources told Bloomberg, putting pressure on the French telecommunications company to increase its offer.

NEW YORK (TheStreet) -- T-Mobile US (TMUS) plans to turn down Iliad's $15 billion deal proposal, sources told Bloomberg, putting pressure on the French telecommunications company to increase its offer.

Iliad said on July 31 it offered $15 billion in cash for 56.6% of T-Mobile.

Sprint S, controlled by Softbank (SFTBF), has been preparing an offer worth about $40 a share, sources said, Bloomberg noted.